Such a guide rail is used for rotation protection of valve tappets, which are typically embodied as roller tappets of a push rod valve train. Simultaneously, they can be used as installation aids for the valve tappet, in that the guide rail and the valve tappets form one structural unit that is protected from loss during transport and installation, so that several valve tappets can be installed simultaneously into the tappet guides of the internal combustion engine.
From DE 101 63 411 A1, which is considered a class-forming invention, a sheet-metal guide rail emerges, which has an especially flat shape and therefore is superior in terms of overall height in comparison with guide rails made from plastic, which are also disclosed in this publication. Plastic guide rails can indeed be produced economically as one-piece injection-molded parts and are also exceptionally well suited, through their own deformation, for equalizing tolerance-related positional deviations in surfaces that are part of the tappet guide system due to their relatively low material stiffness. These surfaces include the tappet guides, the flattened sections of the guide rail, as well as the key surfaces of the valve tappet. Nevertheless, even this low material stiffness must be compensated by increased dimensional stiffness in the longitudinal direction of the valve tappet, so that the overall height of such guide rails can exceed the available installation space in modern internal combustion engines with compact constructions.
While the guide rail proposed in the cited publication, which is composed of sheet metal with a flat overall height, exhibits sufficient dimensional stiffness in the longitudinal direction of the valve tappet, disadvantageously it can also deform only slightly orthogonal to its longitudinal extent and to the longitudinal axis of the valve tappet. The cause here is essentially the relatively high material stiffness of the sheet metal in connection with the high geometrical moment of inertia of the guide rail in this direction. The flattened sections extending parallel to the longitudinal extent of this guide rail are indeed wider than the key surfaces of the valve tappet and therefore permit a free alignment of the valve tappet parallel to the flattened sections; however, compensation of tolerance-related positional deviations is made considerably more difficult due to the high dimensional resistance of the guide rail orthogonal to the flattened sections.
In addition, the flattened sections in the receptacle spaces for the valve tappets are embodied very low and are therefore exposed to a considerable risk of wear as metallic surfaces in their function as rotation protection for the valve tappet. Theoretically this risk of wear can be minimized by subjecting the guide rail to a heat treatment for surface hardening of the flattened sections, but this heat treatment can lead to an impermissibly high dimensional deformation of the guide rail. The alignment errors of the flattened sections with reference to the key surfaces of the valve tappet, in association with such dimensional deformation, can then in practice increase the wear susceptibility of the flattened section despite surface hardening, because the valve tappet mounted in the tappet guides become clamped under the application of considerable transverse forces on the key surfaces between the flattened sections due to the high dimensional stiffness of the guide rail. Simultaneously, it has proven to be extraordinarily difficult to keep this dimensional deformation within tolerable limits in the production of the guide rails in a reliable process, so that there is always the risk of an increased rejection rate and consequently higher costs per piece for the guide rail.